The invention relates to forming a novel calender roll apparatus of the type used in web-processing to assure an excellent finish to the web being subjected to the calendering action.
Although calendering of some webs, e.g. paper, may be relatively non-critical operations, there are many problems involved in critical calendering operations, e.g. those in which high-quality magnetic tape is calendered.
In such calendering, the magnetic tape web is pressed between two rolls. The rolls are being pressed together by forces of the order of 50,000 pounds. The coating is densified and smoothed by this process. There are two general categories:
(i) The two rolls are the same material, e.g. steel against steel; PA1 (ii) The two rolls are of different material, e.g. steel against cotton, plastic, rubber, paper, etc. PA1 1. If creased, the material has memory and will come back to its original shape on heating of the calender roll. PA1 2. The basic mechanical and thermal properties of these alloys fit the application in an extraordinary way. They are softer than the steel roll but much harder than the other compliant roll choices and they are non-magnetic. They have good thermal conductivity far better than the thermal conductivity of plastic, paper or felt rolls. PA1 3. The NITINOL alloy has extraordinary resistance to corrosive attack. The copper oxide, or lead oxide, associated with other soft metals will not occur. The material does not crack as the result of stress corrosion.
In the first situation, the problem is that the steel rolls must be very carefully made, to be sure that they give uniform pressure against the tape surface. Some manufacturers will keep their spare rolls turning 24 hours a day to avoid sag caused by gravity deforming the rolls. Such deforming can cause alternately smooth and rough surfaces as the roll is rotated against the sheet being calendered. Also, any crease or irregularity in the web can so damage a steel roll, that it must be turned down, chrome plated again, and polished. The advantages of steel rolls on calenders is their ability to withstand high pressures in the nip and better heat transfer.
In the second situation, the compliant roll solves many of the mechanical problems of alignment and careful tolerances. Other problems are presented by compliant-surfaced rolls. Such rolls, typically of paper and cotton, are not very strong, so nip pressures must be low. They break down and have to be reground and replaced. A crease in the web will mar the surface of the roll and it will have to be reground. Plastic-faced rolls have been tried and they are successfully used. However, advantageous use of plastic is limited to lower operating temperatures. As the calender turns, the deformation of the compliant roll manifests itself in generating of heat. Plastic and cotton are poor materials for use in removing such heat; they have low thermal conductivity. The result is that the heat builds up at the edges of the roll and the roll gets even softer at such cites, causing more deformation and still more heat build-up along the edges. The edge of the web may reach 40.degree. F. or 50.degree. F. above the middle of the roll. Non-uniform expansion of the roll and non-uniform webs result.
Soft metal surfaces have been tried for complaint rolls. This causes lots of problems because the surfaces are usually not corrosion resistant under plant operating conditions. Again, a crease in the web being processed "wipes out" the roll and necessitates regrinding and refinishing. In general, it may be said that critical calender operations in the prior art relating to magnetic tape use two to five nips with one relatively compliant roll at each nip.
Another aspect of earlier art, one heretofore unrelated to calender design, is the existence of certain titanium/nickel alloys. These materials have been called NITINOL materials because of their nickel and titanium content and the fact that they were developed at the Navel Ordinance Laboratories. The material has been very expensive, on the order of 100 to 1,000 times as expensive as metals. Its use has been heretofore restricted to such applications as small, highly-critical connections for electrical applications such as those required by the U.S. Navy to operate at 6,000 psi or 3,000 psi in temperature of 65.degree. F. to 575.degree. F. Another application which may have become commercial is the use of such materials in formation of dental braces. Various such materials, or related materials, are disclosed in, or discussed in, U.S. Pat. Nos. 3,352,650; 3,174,851; 3,558,369; and 3,660,082.
Still another aspect of earlier art, an aspect not heretofore related to the aforesaid alloy art, is the manufacture of magnetic tape. Of course, it was well known that one should calender tape and that one had to refurbish calender rolls occasionally. However, the value of consistently good calender performance was not believed to have been fully appreciated. Yet, it is an important aspect of this invention that such performance is recognized to be very valuable and to justify the relatively high cost of the aforesaid NITINOL materials in calender applications.
It is emphasized that this section on the prior art is prepared in hindsight with full knowledge of the invention disclosed below. Thus, no pre-existing relationship of disparate elements of the prior art is to be construed from this section.